The carbonylation of methanol produces acetic acid:
Prior to 1970, acetic acid was made using a cobalt catalyst. A rhodium carbonyl iodide catalyst was developed in 1970 by Monsanto. The rhodium catalyst is considerably more active than the cobalt catalyst, which allows lower reaction pressure and temperature. Most importantly, the rhodium catalyst gives high selectivity to acetic acid.
One problem associated with the original Monsanto process is that a large amount of water (about 14%) is needed to produce hydrogen in the reactor via the water-gas shift reaction
Water and hydrogen are needed to react with precipitated Rh(III) and inactive [Rh4(CO)2] to regenerate the active Rh(I) catalyst. This large amount of water increases the amount of hydrogen iodide, which is highly corrosive and leads to engineering problems. Further, removing a large amount of water from the acetic acid product is costly.
In the late '70s Celanese modified the carbonylation process by adding lithium iodide salt to the carbonylation. Lithium iodide salt increases the catalyst stability by minimizing the side reactions that produce inactive Rh(III) species and therefore the amount of water needed is reduced. However, the high concentration of lithium iodide salt promotes stress crack corrosion of the reactor vessels. Furthermore, the use of iodide salts increases the iodide impurities in the acetic acid product.
In the early '90s, Millennium Petrochemicals developed a new rhodium carbonylation catalyst system that does not use iodide salt. The catalyst system uses a pentavalent Group VA oxide such as triphenylphosphine oxide as a catalyst stabilizer. The Millennium catalyst system not only reduces the amount of water needed but also increases the carbonylation rate and acetic acid yield. See U.S. Pat. No. 5,817,869.
One challenge still facing the industry is that lowering water concentration in the methanol carbonylation increases the formation of hydrocarbon impurities such as alkanes and aromatics. Methods for removing alkanes from acetic acid are known. For instance, U.S. Pat. No. 4,102,922 discloses an alkane removal method. According to the '922 patent, a slip stream from the heavy phase which comprises methyl iodide, acetic acid, water and alkanes is fed to an alkane distillation column with an overhead temperature of about 75° C. and a bottoms temperature of about 142° C. The bottoms temperature is run significantly higher than the overhead in order to minimize methyl iodide loss to the bottoms stream. The overhead of the alkane distillation, comprising mainly methyl iodide, is recycled to the reaction section. The bottoms stream comprising about 50% acetic acid and about 40% alkanes is removed from the system as waste. One problem associated with this method is that due to the high bottoms temperature, low boiling alkanes such as 2-methylpentane come with the overhead methyl iodide. This results in a build up of the low boiling alkanes in the reaction system as the overhead methyl iodide is recycled into the carbonylation reaction.
A new method for removing alkanes and other hydrocarbon impurities from the acetic acid production process is needed. Ideally, the method can effectively remove both high boiling and low boiling hydrocarbon impurities from the acetic acid production process.